Fuel element for atomic reactors



w. BLOMEYER ETAL 3,184,393

May 18, 1965 United States Patent 3 Claims. (51. 176-82) The invention relates to a fuel element for atomic reactors, with a heat-transmitting bond between the fuel and the can, and to a process for the manufacture thereof.

Fuel elements for reactors cooled by organic materials or water are provided with a heat-transmitting bond between the fuel, such as uranium, and an aluminum can by means of a nickel soldering layer. It is known to connect the components by soldering of a galvanically deposited nickel layer under pressure. For example, the can is pressed on to the nickel-coated uranium under a gas pressure of over 500 atmospheres, so that at a temperature of 540 C. the nickel forms a metallic bond with the aluminum on one side and the uranium on the other. But this procedure entails certain disadvantages. During the subsequent operation of the reactor, in accordance with the thermal load, a further Al Ni diffusion takes place, which results in a further conversion of nickel into Ni Al through NiAl Ni Al and NiAl. If the whole nickel layer alloys with the aluminum, aluminum is given up from the nickel-aluminum compounds to the uranium, because the enthalpy of formation of uranium-aluminum compounds is greater than that of nickel-aluminum compounds. In order to prevent, with any assurance, the formation of the undesirable uranium-aluminum compounds, a sufficiently thick layer of pure nickel must be preserved between the aluminum-nickel and the nickeluranium solder zones. This requires that the soldering conditions, such as pressure, temperature and distribution of heat, and the uniformity of the galvanic nickel layer, must be maintained very exactly. Also, at 500 C., the rate of growth of the Ni Al phase is around 1X 10- cm./h., so that the combination of aluminum with nickel proceeds so rapidly that even a relatively thick nickel layer cannot prevent in the course of time the formation of uranium-aluminum compounds.

Another noticeable disadvantage of the known soldering procedures is the expense, since the high pressures required involve the use of quite expensive autoclaves and heaters.

The primary object of the present invention is to provide a fuel element which overcomes the disadvantages of the prior art structures, in which the danger of diffusion of aluminum into the uranium is substantially avoided, and which is less expensive to produce, being formable at a substantially lower pressure and with smaller quantities of expensive ingredients.

It has been found that the disadvantages of the known fuel elements can be avoided if a diffusion-arresting layer is introduced into the nickel layer. A layer of chromium 5 thick is especially suitable. Such a chromium layer blocks the spread of the diffusion of aluminum into the nickel, and makes it possible to raise the soldering temperature to 600 C., so that a pressure of only 80 atmospheres is needed. The arresting effect of the chromium depends on the stronger bonding of aluminum atoms in the lattice of nickel aluminum compounds in comparison with the bonding of aluminum atoms in chromium-aluminum compounds. The rate of Ni+ Cr diffusion at the temperatures involved is very small and produces no intermetallic compounds. A thin, homogeneous chromium layer olfers enough protection. It is also ade- Patented May 18, 1965 "ice quate to assure a strong connection between the solder layers.

A metallic bond between uranium or uranium alloys and aluminum or aluminum alloys produced in this way has a strength which is greater than that of conventional canning materials of this type. The product has the further advantage over known pure nickel solder arrangements of having a lower neutron absorption through the solder layers, because the thickness of nickel can be substantially reduced and the thin chromium layer results in no increase of the capture cross-section.

Further objects and advantages of the invention will appear more fully from the following description especially when taken in conjunction with the accompanying drawing which forms a part thereof.

The drawings shows in cross-section a part of a fuel element embodying the invention.

In the drawing, 2 is a part of a uranium or uranium alloy fuel rod. On the outside of the rod and bonded to it is a layer 4 of nickel, which is 6, thick, then a layer 6 of chromium 5;]. thick, a nickel layer 8 which is 4;]. thick, a copper layer 10 which is 1 thick, and a can 12 of aluminum or aluminum alloy. The copper layer can be omitted, but has certain advantages to be discussed below.

Preferably, the thickness of the various layers is within the following ranges:

The copper layer does not tend to form compounds with the resulting multiple-ingredient systems and has an adequate solubility for the separate components.

The various layers can be applied as galvanic precipitates in a vacuum with heating to remove gases. After the application of the aluminum can and a temperature treatment at 600 C. under a pressure of atmospheres for 30 minutes, there is a good mechanical bond between the fuel and the can, which after long exposure to a temperature of 500 C. shows no formation of aluminumuranium compounds.

The galvanic layers can also be deposited on the inside of the can. Suitable can materials are 99.5% aluminum; aluminum-magnesium alloys with 3 to 5% magnesium, and sintered aluminum powder, or other aluminum-base alloys used in the reactor field. This is also true of the fuel, including various types of uranium-rich alloys. The solder layers can be deposited in other ways, as by decomposition of icarbonyls or vaporization at high vacuum.

While we have described herein some embodiments of our invention, we do not intend to limit ourselves thereby except within the scope of the claims hereto or hereinafter appended.

We claim:

1. In a fuel element for atomic reactors comprising a fuel body consisting essentially of uranium, a can consisting essentially of aluminum, a first nickel solder layer bonded to said body, a second nickel solder layer bonded to said can, and a layer consisting essentially of chromium between and bonded to both said nickel layers for retarding diffusion of aluminum into the uranium.

2. In an element as claimed in claim 1, the thickness of said layers being within the following ranges:

First nickel layer 5 to 7 Second nickel layer 4to 6 Chromium layer 3 to 5 3. In a fuel element for atomic reactors comprising a fuel body consisting essentially of uranium, a can consisting essentially of aluminum, a first nickel solder layer bonded to said body, a layer of chromium bonded to said 3 4 first nickel layer for retarding diffusion of aluminum into 2,394,320 7/59 Gurinsky et 7 the uranium, a second nickel layer outside of and bonded 2,928,168 3/ 60 Gray 176-91 X to the chromium layer, and a layer of copper outside of 2,956,000 10/60 Kendall et -83 and bonded to the seco'nd nickel layer and bonded to the 2,967,811 1/61 Flint 176-82 inside of the can. 5 2,969,309 1/61 Finniston et a1. 17.6-82 e 2,990,352 6/61 Finniston'et a1. 176-82 Refer n s i e by the min r 3,004,906 10/61 Binstock 176-82 UNITED STATES PATENTS 2,854,737 10/58 y 176 67 CARL D. QUARFORTH, Primary Exammer.

2 54 73 10 5 Gray 17 7 1 OSCAR R. VERTIZ, Examiner. 

1. IN A FUEL ELEMENT FOR ATOMIC REACTORS COMPRISING A FUEL BODY CONSISTING ESSENTIALLY OF URANIUM, A CAN CON/ SISTING ESSENTIALLY OF ALUMINUM, A FIRST NICKEL SOLDER LAYER BONDED TO SAID BODY, A SECOND NICKEL SOLDER LAYER BONDED TO SAID CAN, AND A LAYER CONSISTING ESSENTIALLY OF CHROM- 